I picked up the XBox version today at the Salisbury, MD store! They still had one left. The thing is, it's not in the glass case but on the endcap where they keep all the discontinued games (the ones that aren't in clearance yet).
Last week, I remember seeing another copy of Burnout 3 for XBox at a different Target in PA. It was also in the endcap rather than the glass case. This is just a theory but maybe that's why people are having trouble finding the XBox version - since it's already been moved out of the glass cases. Has everyone been checking the endcaps as well as the glass cases?